1. Field of the Invention
This invention relates generally to the measurement of relative flow between a fluid and a defined volume, such as a pipe, and more particularly, to the measurement of fluid flow and composition by nuclear or electron magnetic resonance.
2. Description of the Prior Art
The idea of studying flow by magnetic resonance dates back to the work of the early pioneers as described, for example, in Mansfield, P; Morris, P. G.; xe2x80x9cNMR Imaging in Biomedicinexe2x80x9d; Advances in Magnetic Resonance, Supplement 2; 1982; Academic Press, Inc. Orlando 32887; p.235 section 7.3.5. Prior art devices for flow measurement or flow mapping rely on two well-known methods viz. xe2x80x9cTime-of-Flightxe2x80x9d of saturated or unsaturated spins or xe2x80x9cPhase-Encodingxe2x80x9d by application of a gradient field along the direction of flow. (Cho, Z. et. al.; xe2x80x9cFoundations of Medical Imaging;xe2x80x9d John Wiley and Sons, Inc., New York, 1993, p374-386.) Exemplary of the xe2x80x9cTime-of-Flightxe2x80x9d method is U.S. Pat. No. 4,782,295 to Lew and of the xe2x80x9cPhase-Encodingxe2x80x9d method is U.S. Pat. No. 5,532,592 to Maneval. Analysis of chemical composition by chemical shift is discussed in xe2x80x9cPrinciples of Magnetic Resonance,xe2x80x9d third edition chapter 4, by Slichter, C. P., Springer-Verlag, N.Y. 1989.
One preferred aspect of the present invention provides a universally applicable simplified method to non-invasively measure the mean value of, or to map the velocity profile of, the various domains of flow based on the dwell time of flowing spins within a defined space containing a constant uniform H1 Larmor radio frequency excitation field.
Another preferred aspect of the invention provides a method to measure or map the signal received from moving spins within a defined space in the continuous presence of the H1 Larmor radio frequency excitation field by periodically phase modulating the H0 strong main magnetic field by a periodic gradient field so as to cause the spins to emit a line or band spectrum, centered at the Larmor frequency, whose sideband amplitudes are known functions of the amplitude of the center-band Larmor frequency signal emitted by the spins, said emitted center-band Larmor frequency signal amplitude being a known function of the dwell time of the spins within a defined space within the H1 Larmor excitation field.
Another preferred aspect of the invention provides a method to continuously measure the very weak sidebands of the emitted signal from the phase modulated spins in the presence of the very strong H1 central Larmor field by demodulating and then cross-correlating the received signal with integral multiples of the phase modulating frequency of the periodic gradient field.
In another preferred aspect of the invention, the amplitude of the phase modulating H0 field is spatially ordered to permit the spatial mapping of the dwell time of the spins within a defined volume within the H1 excitation Larmor field.
In another preferred aspect of the invention, the pulsed Larmor radio frequency fields and pulsed gradient fields are eliminated, thereby reducing or eliminating eddy currents, transients, and Gibbs truncation artifacts.
Another preferred aspect of the invention provides a method to measure or map the velocity or perfusion vector of the spins from the measurement of, or the map of, the dwell time of the spins within the known geometry of a defined portion of the H1 Larmor excitation field, said known geometry being defined by a receiver coil preferably wound orthogonal to the H1 Larmor excitation field coil so as to substantially decouple the noise from, and the signal from, the H1 Larmor excitation field.
Another preferred aspect of the invention is to provide a measure of, or a map of, the flow velocity or perfusion vector within the known geometry of a defined portion of the H1 Larmor excitation field constructed from measurements dependent on the dwell time of the spins in a defined portion of the H1 Larmor frequency excitation field as measured with the known adjustable strength of that H1 Larmor frequency excitation field, and not significantly dependent on the unknown T1 spin-lattice, the unknown T2 spin-spin, the unknown D diffusion, or on other unknown parameters affecting spin magnetization, spin diffusion, or spin coherence. These unknown parameters affect the signal-to-noise of the measurements of this invention, but not significantly the defined end point of these measurements, according to this invention.
A further preferred aspect of this invention is to perform a simultaneous chemical and physical analysis of the flowing material.
A further preferred aspect of this invention is a flow meter for performing one or more of the above methods.